1. Field of the Invention
The present invention relates to a joint member used in welding square tubular materials made of aluminum or aluminum alloy (hereinafter generically referred to as aluminum material) to form a welded structure serving as a body frame or the like for an automobile such as a truck.
2. Description of the Related Art
According to a conventional method of forming body frames for automobiles, such as trucks, or transport planes, formed steel plates are homed at their end portions, and the homed portions are joined by resistance spot welding for reinforcement. The thus-assembled monocoque structure is used as a typical body frame for an automobile such as a truck.
This method has an advantage that assembly work can be readily automated, but has disadvantages that an assembled structure has many overlapping portions and wasteful portions and is rather bulky, and that formed members have complicated shapes with a resultant increase in weight.
By contrast, if formed members are butt-welded by arc welding or beam welding, an assembled structure will not become bulky. However, weld quality depends significantly on the butt accuracy between members to be welded. For example, if any gap is formed between members to be welded, weld quality is impaired significantly. Accordingly, jigs must be prepared to maintain members in an assembled position. Also, members themselves must be machined to high accuracy. Thus, assembly work is difficult to automate.
Recently, in order to reduce the weight of a car body, an attempt has been made to manufacture a car body from aluminum alloy. Since aluminum material is low in weldability, welding must be performed at thousands of points to assembly a monocoque structure. Therefore, consumption of electrodes is large. Also, aluminum material has a problem that formability is poorer than that of steel.
Thus, there is proposed a space frame structure in which a body frame of an automobile such as a truck, a transport plane, or the like is supported by a hollow pipe frame. FIG. 13 is a schematic view showing a space frame structure proposed for application to a typical truck. As shown in FIG. 13, square tubular aluminum sections 51 are assembled into the shape of a truck cab and welded, so that square tubular aluminum sections 51 are mutually fixed. Thus a so-called space frame structure 50 is formed. The space frame structure 50 is covered with sheet metal to complete a body of the truck.
In connection with this method of assembling square tubular aluminum sections into a body frame of an automobile, there has been proposed a method of joining sections via a connection member manufactured through casting or forging (Japanese Patent Application Laid-Open (kokai) No. 60-135375).
In a frame assembly of various kinds of structures utilizing tubular materials, when each joined portion of the assembled frame requires a strength greater than that of the frame, the joined portion is usually reinforced in the following fashion: a reinforcement plate, which is shaped and sized so as to fit a connection angle at the joined portion and to meet a required strength criterion, is applied onto the outer surface of the frame at the joined portion and is then welded to the frame.
However, this conventional method using connection members has a drawback that cost of manufacture is increased due to the complicated shape of connection members and an increased number of component members.
In this connection, the applicant of the present invention has proposed a method of manufacturing a welded structure through direct welding between sections without using connection members (Japanese Patent Application No. 8-143152, not laid open). In this earlier technology, in order to obtain good weld when sections are directly welded, end portions of sections to be welded are cut obliquely and butted against each other such that a weld bead bisects a connection angle (an angle formed by sections to be welded).
However, the work for obliquely cutting and welding the end portions of sections involves the following problems. In a process of assembling a frame wherein sections are butted against each other with a predetermined angle formed therebetween and fixed temporarily by clamping or welding, sections are unlikely to be held with a predetermined connection angle formed therebetween, and butt end portions are likely to be dislocated. As a result, setting and welding become troublesome and take time.
Also, when a joined portion is to be reinforced, after assembling a frame by welding, reinforcement plates are welded to the frame, i.e. two welding steps are involved, welding for assembly and welding of reinforcement plates. Thus, welding is rather complicated. Also, if a weld bead is present on the frame surface where a reinforcement plate is to be welded, the reinforcement of weld must be ground off, involving an additional step and additional labor.
Further, the shape and size of reinforcement plates must be designed from one reinforcement plate to the other so as to establish required characteristics for each joined portion to be reinforced. This increases design cost and requires reinforcement plates to be cut and pressed one by one according to their individual design, resulting in increased cost of manufacture.